Recently, a mobile terminal on which an image pickup apparatus employing a solid-state image pickup element such as a CCD type image sensor or a CMOS type image sensor is mounted are coming into wide use. As for an image pickup apparatus to be mounted on such a mobile terminal, those employing an image pickup element with a large number of pixels have been supplied into the market in order to obtain images with higher image quality. Image pickup elements with a larger number of pixels were large in size. However, corresponding to the recent progress of pixels in terms of minuteness, image pickup elements have been becoming small-sized recently.
An image pickup lens used for such the image pickup elements with minuteness is required to have high resolving power. However, the resolving power is limited depending on F-number, and it is becoming difficult to give sufficient performance with a conventional art with F-number of about F2.8. Therefore, image pickup lenses with excellent lens speed of about F2, suitable for a small-sized image pickup lens with a large number of pixels has been required, because a lens with smaller F-number and more excellent lens speed exhibits higher resolving power. As an image pickup lens for such the use, there has been proposed a five-element image pickup lens that can make the aperture diameter larger and make its performance higher than a lens having a three-or-four-element structure.
As a five-element image pickup lens, there has been known an image pickup lens composed of, in order from the object side, the front group composed of the first lens with positive or negative refractive power and the second lens with positive refractive power, an opening aperture, and a rear group composed of a third lens with negative refractive power, a fourth lens with positive refractive power, and a fifth lens with positive or negative refractive power (For example, see Patent Literature 1 and Patent Literature 2).
Further, there has been known a four-element image pickup apparatus with lens speed of about F2 (For example, Patent Literature 3).
Citation List
Patent Literature
Patent Literature 1: JP-A No. 2007-279282
Patent Literature 2: JP-A No. 2006-293042
Patent Literature 3: JP-A No. 2007-322844